Such illumination systems are known per se. They are used, inter alia, in luminaires for general lighting purposes, for example, for office or shop lighting or, for example, as shop window lights. Alternatively, these illumination systems are used for lighting (transparent or semi-transparent) plates of glass or (transparent) synthetic resin on which items, for example, jewelry are displayed. The illumination systems are also used for illuminating advertising boards, for example, advertising boards illuminating a partially transparent image from the back of the image.
The known illumination systems may also be used as light sources in backlighting systems in (picture) display devices, for example, for TV sets and monitors. Such illumination systems are particularly suitable for use as backlighting systems for non-emissive displays such as liquid crystal display devices, also denoted LCD panels, which are used in (portable) computers or (portable) telephones.
Recently, illumination systems comprising a light source having a substantially Lambertian emission profile, for example, light-emitting diodes (further also referred to as LEDs) have been more frequently used in general lighting applications, such as luminaires. Use of LEDs in luminaires has the advantage that the LEDs have a relatively high efficiency. Furthermore, there is a vast range of available LEDs for use in general light applications, offering excellent freedom of design with respect to emitted colors and aesthetic design of the luminaire. However, use of LEDs has the drawback that the light-emitting surface is typically relatively small and that light emitted from this relatively small surface has a high intensity. To prevent glare, it is important to shield a user of the luminaire from this light-emitting surface. Glare results from excessive contrast between bright and dark areas in the field of view. Glare can result, for example, from directly viewing a filament of an unshielded or badly shielded light source. Particularly when LEDs are used, direct view into the LEDs by a user near the luminaire should be prevented so as to reduce glare and improve the user's visual comfort. A luminaire comprising LEDs as a light source is known from, for example, WO 2007/054889. In this luminaire, the light emitted by the LEDs is emitted via a back-reflector so as to reduce glare and improve the uniformity of the light emitted from the luminaire. The known luminaire has the drawback that it has a relatively large height.